Activity 2.6 NCERT Class 10 Science, Chapter 2 Acids, Bases, and Salts.
This activity asks us to add a few drops of phenolphthalein into dilute sodium hydroxide and see what happens when we add dilute acid.
Observation: A time comes after adding acid that pink solution of phenolphthalein disappears. Now again if we start adding sodium hydroxide it becomes pink.
Phenolphthalein is a colourless reagent. It turns the solution into pink at basic PH. So, It is used in an acid-base titration to see the endpoint. Like here adding acid neutralises the base present in the test tube. Now at the endpoint of the reaction, the addition of an extra drop of acid makes the solution acidic. As a result, the pink colour disappears.
Situation reverse when we add a base to this acidic solution. As a result, the solution gets own previous pink colour.
Inference/conclusion: A change in colour of PH indicator can indicate the completion of a reaction.